In modern telecommunication industry based on optical fiber interfaces, the cost of optoelectronics components is a key factor for the deployment of the next generation optical links. As response to this challenge, the photonic integrated circuits (PIC) technology is regarded as very promising approach because it offers the benefit of combining different functions such as light generation, amplification, processing and detection in most cost-efficient way. At the same time, in any PIC development the key difficulty is the accommodation of active components having different bandgaps, such as lasers, modulators or detectors on a single chip. One integration technique that achieves this aim places the device waveguides, one under another in a vertical epi-stack. In this case, however, the design of the PIC is complicated, from one side, by the conflicting requirements of layer isolation, so that device functions can be optimized, and from another side, by the need of communication between the layers, so that optical circuits can be made.
Usually, to address these difficulties, a passive guide is inserted between the active devices to achieve isolation, and then lateral tapers are used at each guiding layer so that the light from the upper active waveguide is transferred to the intermediate passive waveguide followed by another transition into the lower active waveguide. Such approach is extensively used in the previous art and described in a number of patents (e.g. S. Saini et al, “Resonantly coupled waveguides using a taper”, U.S. Pat. No. 6,310,995, S. Forrest, M. Gokhale and P. Studenkov “Twin waveguide based design for photonic integrated circuits”, U.S. Pat. No. 6,381,380, S. Forrest et al, “Photonic integrated detector having a plurality of asymmetric waveguides” U.S. Pat. No. 6,330,378).
The specific feature of this approach is that the both upper and lower waveguides only interact with the middle waveguide, the middle waveguide bridges the two waveguides, at the same time isolates them as is illustrated in FIG. 1 a). While this strategy addresses the needs of both isolation and communication, the intermediate passive waveguide must have the strength, i.e., effective index, that lies between the upper and lower active waveguide ones. The requirement on the middle waveguide strength introduces additional constraints to the optimization of the active waveguides. For example, it imposes limitations on the achievable confinement factor of the device waveguide, which is an extremely important parameter for efficient lasers and modulators.
The design approach existing in the prior art assumes that light is transferred only between adjacent guiding layers of the vertical stack what in turn implies the geometry that the waveguide tapers on different levels should be separated in space; otherwise one transfer would be in conflict with another one. Such geometrical constraint inevitably increases the chip's footprint and leads to higher chip's cost.
Therefore, there is a need in the art of multi-guide PIC technology to lift limitations of light interaction only between adjacent waveguides; there is a need to provide solution when light signal could be efficiently redirected from one vertical layer of the stack into any other desirable vertical waveguide.
Therefore, there is a further need in the art to provide solution of (de)multiplexing signals of different wavelength when light signal of desired wavelength could be efficiently redirected from one vertical layer of the stack into any other desirable vertical waveguide.
Therefore, there is a further need in the art to provide solution of power splitting when the power of the light signal being initially localized in one particular waveguide of the vertical stack could be efficiently redistributed between several vertical waveguides of the stack in desired ratio.
Generic case of light propagation in a planar structure of three or more coupled optical waveguides, illustrated in FIG. 2, is well studied in the literature [E. Paspalakis, “Adiabatic three-waveguide directional coupler,” Opt. Commun. 258, 30-34 (2006); S. Longhi, G. Della Valle, M. Ornigotti, and P. Laporta, “Coherent tunneling by adiabatic passage in an optical waveguide system,” Phys. Rev. B 76, 201101 (2007); H. S. Hristova, A. A. Rangelov, S. Gu'erin, and N. V. Vitanov “Adiabatic evolution of light in an array of parallel curved optical waveguides” Phys. Rev. A 88, 013808 (2013)]. In the theory which is based on the slowly varying envelope approximation, the spatial evolution of the amplitudes of the optical modes an(z), with n=1, 2, 3, is described by the following set of coupled differential equations
                                                        -              i                        ⁢                                          ⅆ                                  a                  1                                                            ⅆ                z                                              =                                                    β                1                            ⁢                                                a                  1                                ⁡                                  (                  z                  )                                                      +                                                            k                  12                                ⁡                                  (                  z                  )                                            ⁢                                                a                  2                                ⁡                                  (                  z                  )                                                                    ,                            (        1        )                                                                    -              i                        ⁢                                          ⅆ                                  a                  2                                                            ⅆ                z                                              =                                                    β                2                            ⁢                                                a                  2                                ⁡                                  (                  z                  )                                                      +                                                            k                  21                                ⁡                                  (                  z                  )                                            ⁢                                                a                  1                                ⁡                                  (                  z                  )                                                      +                                                            k                  23                                ⁡                                  (                  z                  )                                            ⁢                                                a                  3                                ⁡                                  (                  z                  )                                                                    ,                            (        2        )                                                                    -              i                        ⁢                                          ⅆ                                  a                  3                                                            ⅆ                z                                              =                                                    β                3                            ⁢                                                a                  1                                ⁡                                  (                  z                  )                                                      +                                                            k                  32                                ⁡                                  (                  z                  )                                            ⁢                                                a                  2                                ⁡                                  (                  z                  )                                                                    ,                            (        3        )            where, βn with n=1, 2, 3, is the constant propagation coefficient of the nth waveguide and knm(z), with n,m=1, 2, 3 is the variable coupling coefficient between the waveguides n and m. It was found that when the coupling between planar waveguides has a specific dependence on the propagation coordinate z, as illustrated in FIG. 3, then the transfer from the outermost waveguide into another one occurs via so-called tunneling mechanism. A distinctive feature of this mechanism is that light does not delay in the middle waveguide and goes directly from one outermost waveguide into another one. This coupling mechanism has been termed as a coherent tunneling by adiabatic passage (CTAP) [E. Paspalakis, “Adiabatic three-waveguide directional coupler,” Opt. Commun. Vol. 258, 30-34 (2006); S. Longhi, G. Della Valle, M. Ornigotti, and P. Laporta, “Coherent tunneling by adiabatic passage in an optical waveguide system,” Phys. Rev. B Vol. 76, pp. 201101-201105 (2007).].
FIG. 4 presents further details of the CTAP mechanism, showing that light stays mainly in the modes of waveguides #1 and #3 without significant excitation of the mode in waveguide #2.
CTAP light exchange mechanism may be extended to the larger number of coupled waveguides as it is demonstrated in the work by H. S. Hristova, A. A. Rangelov, S. Gu′erin, and N. V. Vitanov “Adiabatic evolution of light in an array of parallel curved optical waveguides” Phys. Rev. A Vol. 88, 013808-013811 (2013). FIG. 5 shows light exchange between outermost waveguides in the four waveguide array. It is seen that light goes from one outermost waveguide to another outermost waveguide with only partial excitation of intermediate waveguides.
The CTAP formalism involving coupling between three states described by equations (1)-(3) is relevant to many physical applications in different fields of science and technology; one such application is controlling the population of quantum states of atoms and molecules by external laser pulses as it is described in the US patent by S. Nakatuura, K. Ichimura and H. Goto “Operating method for stimulated Raman adiabatic passage and operating method for phase gate” U.S. Pat. No. 8,488,232 B2.
The CTAP concept has been applied to Silicon-on-Insulator (SoI) platform with the result that light modes can be efficiently transferred between two lateral waveguides with help of third mode which is the silicon slab mode or the supermode of the full structure. [A. P. Hope, T. G. Nguyen, A. D. Greentree, A. Mitchell, “Long-range coupling of silicon photonic waveguides using lateral leakage and adiabatic passage” Opt. Express, Vol. 21, pp. 22705-22716 (2013), L. Socci, V. Sorianello, and M. Romagnoli, “300 nm bandwidth adiabatic SOI polarization splitter-rotators exploiting continuous symmetry breaking” Opt. Express, Vol. 23, pp. 1926149271 (2015).]. However, the planar nature of the SoI platform does not allow application of three-dimensional potential of the CTAP approach, whereas the semiconductor multi-layered structures are very good candidate for realization of all CTAP mechanism benefits.
While the theoretical background of efficient light transfer between different waveguides in the array is well elaborated, the application of the CTAP theory to the field of the vertical photonic integration is still missing. The current invention exploits the concept of controlled tunneling waveguide integration (CTWI) to develop multiple vertical/lateral waveguide platform suitable for cost-efficient PIC fabrication.